a square piece of tin 31cm and20cm is to be made into a box without top,by cutting a square from each corner and folding up the flaps to form the box.what should be the side of the square to be cut off so that the volume of the box is the maximum possible?
v = x(31-2x)(20-2x)
dv/dx = 12x^2 - 204x + 620
dv/dx=0 when x=3.96